Thousands of people massed in Thessaloniki to rail against proposed legislation that includes fresh tax increases as well as yet more wage, pension and job cuts. Parliament was expected to vote on the bill Thursday.

Sakis MitrolidisAFP/Getty Images

Anger over proposed new austerity measures boiled over in Greece on Wednesday as unions shut down the country with what one newspaper called "the mother of all strikes."

Flights were grounded, and state offices and shops were shuttered on the first day of a 48-hour general strike, the biggest organized protest against austerity since the debt crisis began almost two years ago.

In a remarkable display of political expression, tens of thousands of striking workers marched through the avenues of central Athens, chanting anti-capitalist and anti-government slogans. They gathered before Parliament, where lawmakers were debating a controversial austerity bill that would raise taxes and cut public sector pay and pensions, among other things, ahead of a vote Thursday.

International creditors have demanded the reforms before they give Greece its next infusion of cash. Greece says it will run out of money in a month without the $11 billion bailout money from its partners that use the euro, and the International Monetary Fund.

Most of the protesters who converged in central Athens marched peacefully, but some demonstrators tore chunks of marble off building fronts with hammers and crowbars, and smashed windows and bank signs. A gasoline bomb set fire to a presidential guard sentry post outside Parliament. Riot police responded with tear gas and stun grenades that echoed across Athens' main Syntagma Square.

In the city of Thessaloniki, protesters smashed the facades of about 10 shops that defied the strike and remained open, as well as five banks and cash machines. Police fired tear gas and threw stun grenades.

Flights were grounded in the morning but some resumed at noon after air traffic controllers scaled back their strike plan from 48 hours to 12. Dozens of domestic and international flights were still canceled. Ferries remained tied up in port, while public transport workers staged work stoppages but kept buses, trolleys and the Athens metro running to help protesters.

"There are piles of garbage everywhere in Athens. ... And to give you a sense of the anger and how fed up Greeks are with the austerity measures, a garbage strike of this scope some years ago would have triggered fury at the strikers," she said. "Today, even though the Health Ministry has warned of the dangers for public health, there's a very strong sense of solidarity with the garbage collectors."

Civil servants also have taken over several government buildings, including the Finance Minstry. Poggioli said Finance Minister Evangelos Venizelos has been unable to get into his office for the past two weeks, and that the building is pasted with leaflets declaring, "The massacre of our wages will be the nightmare of bankers."

In Parliament, Venizelos told lawmakers Wednesday that Greeks had no choice but to accept the hardship.

"We have to explain to all these indignant people who see their lives changing that what the country is experiencing is not the worst stage of the crisis," he said. "It is an anguished and necessary effort to avoid the ultimate, deepest and harshest level of the crisis. The difference between a difficult situation and a catastrophe is immense."

Although taxpayers want smaller and more efficient government, many now fear that working conditions in Greece will become like those of a Third World country.

Opponents worry the latest austerity package could mean the end of the minimum wage and leave Greeks competing for jobs with below-subsistence wages. And unions have been especially incensed by a provision curtailing collective bargaining rights in the private sector.

"If the economy falls into a deeper recession, state revenues will fall dramatically," former Labor Minister Louka Katseli told MEGA, a Greek TV station. "It's our responsibility to point this out to international lenders."

The government is trying to negotiate with unions and Katseli, who is still in Parliament and has threatened to veto the bill.

But Platon Tinios, an economics professor at the University of Piraeus, said unions have long held sway over political parties, and that restricting their influence will help open up the economy at a crucial time.

"They certainly haven't done much to promote economic growth or competitiveness," Tinios said. "They haven't done very much to get us out of the problem, and you can point to quite a lot of things that they have done to get us into the problem."

He said unions long prevented reforms now seen as inevitable — changes to the bloated pension system, for example, or privatization of state assets. Now Greece is bankrupt and has no choice but to change, he said.

About 3,000 police deployed in central Athens, shutting down two metro stations near Parliament as the protest marches began. Protesters banged drums and chanted slogans against the government and Greece's international creditors.

"We just can't take it any more. There is desperation, anger and bitterness," said Nikos Anastasopoulos, head of a workers' union for an Athens municipality.

Joanna Kakissis and NPR's Sylvia Poggioli reported from Athens for this story, which contains material from The Associated Press.